In the most common cases the propulsion of various ships or equivalent vessels (such as e.g. passenger ships and passenger ferries, cargo ships, barges, oil tankers, ice breakers, off-shore vessels, naval ships etc.) is achieved by a pushing or pulling force produced by a rotatable propeller or several propellers. Separate rudder devices have conventionally been used for steering the vessels.
Conventionally the driving or rotating arrangements for the propellers have been implemented such, that the drive device for the propeller shaft, e.g. a diesel engine, a gas engine or an electric motor is provided inside the hull of the vessel, from which location the propeller shaft extends outside the hull of the vessel through a watertight sealed opening. The propeller itself is located at the other end, i.e. the end extending outside the vessel, of the propeller shaft, which is connected either directly to the motor or to a gearbox, if any. In the majority of all vessels used in water-borne traffic this solution is used for providing the force necessary for their movement.
Prior art is described below with reference to the accompanying drawings, in which:
FIG. 1 shows the structure of a propulsion unit according to prior art,
FIG. 2 shows a prior art propulsion unit with its turning arrangement, and
FIG. 3 shows a prior art arrangement for supplying power to propulsion units.
FIG. 1 shows the structure of a propulsion unit according to prior art. The prior art propulsion unit comprises a motor 1, a propeller 2, and a chamber 3, which is connected to the hull 5 of the ship by a hollow shaft 4. Shipbuilders have lately started to provide vessels with propeller units of the described kind, wherein the motor 1, providing the driving power for the propeller shaft, as well as any gearbox, are disposed inside a special chamber 3, which is located outside the hull 5 of the ship, said chamber 3 being supported rotatably in relation to said hull 5.
The chamber 3, containing the motor 1, is rotatably supported in relation to the hull of the ship by the hollow shaft 4, which is extending through the bottom of the ship. Thus, the unit is turnable relative the hull 5, whereby it, instead of any separate rudder equipment, can also be used for steering the vessel. Among others, the FI patent No. 76977, to the present applicant, discloses such a propulsion unit in more detail. These units are also generally called azimuthing propulsion units, and the present applicant, for example, is offering such azimuthing units under the trade name AZIPOD®.
In addition to the advantages gained from the elimination of a long propeller shaft and separate rudder equipment, these devices have been found to provide essential improvement in the maneuverability of a vessel. The energy economy of the vessel has also been found to become more efficient. In recent years, the use of azimuthing propulsion units in various vessels for water-borne traffic has become more common, and they are assumed to grow increasingly popular.
FIG. 2 shows a prior art propulsion unit with its turning arrangement. The propulsion unit according to prior art comprises a motor unit 6, a propeller 7, a supporting part 8, and a cabling unit 9 and motor units 10, 11 of the turning arrangement. The motor unit 6 and the propeller 7 of the propulsion unit are being supported at the hull of the ship by the supporting part 8. The driving electric power is supplied to the electric motor unit 6 by the cabling unit 9. The motor units 10, 11 of the prior art turning arrangement turn the propulsion unit via a suitable gear set. The prior art propulsion unit also comprises mechanical brake means for reducing and/or limiting the turning speed of the azimuthing propulsion unit.
FIG. 3 shows a power supply arrangement according to prior art for propulsion units. The prior art power supply arrangement for propulsion units comprises engine units 12, generator units 13, electrical power network switching panels 14, transformer units 15 for the propulsion system, frequency converters 16 for the propulsion units, frequency converters 17, 18 for the turning arrangement, propulsion units 19, 20 and a steering system 21.
In the power supply system according to prior art, the engine units 12 produce the power and the generator units 13 convert it to electric energy as input to the electrical power network. In the electrical power network, the electric power is directed, via the switching panels 14 and the transformer units 15 for the propulsion system, to the frequency converters 16 for the propulsion units and the frequency converters 17, 18 for the turning arrangement. The frequency converters 16–18 further establish the driving voltages to the motors for the propellers and the turning arrangements of the propulsion units 19, 20. The task of the steering system 21 is to control the operation of the turning arrangement.
The operating principle of the frequency converter 16–18 is a technique known to a person skilled in the art, and thus it need not be explained here, except by mentioning, that the main general parts of a frequency converter are a rectifier, an intermediate direct-current circuit and an invertor. Presently frequency converters 16–18 are commonly used, for instance, as supply devices for alternating-current motors, while being particularly preferable in various controllable electric drives. Frequency converters called PWM frequency converters (PWM, pulse width modulation), which are based on the pulse width modulation technique and have a so called voltage intermediate circuit, are the frequency converters being utilized the most frequently.
Should one or more of the propellers get damaged, it should be possible to move the vessel for service. The problem is, that should the vessel be moved, the damaged propeller will easily start rotating and causes additional damage to the propulsion device and possibly also to the vessel.
Accordingly, should there occur a break-down in the electric power supply to the motors in the turning arrangement for the propulsion unit, a sudden turning of the propulsion unit might cause additional damage to the propulsion device and possibly also to the vessel. If the propulsion device is free recklessly to turn this way and that, the maneuverability of the vessel is considerably impaired.
In regard to prior art reference is made to U.S. Pat. No. 1,555,244, which presents a solution for dynamic braking of an asynchronous motor, whereby the stator windings are short-circuited in a known manner. Furthermore, in regard to prior art reference is made to U.S. Pat. No. 5,184,049 which presents a solution for reducing the motor stopping time of the optical or magnetic discs by first supplying reverse voltage to and then short-circuiting the stator windings of an ac motor. Furthermore, in regard to prior art reference is made to International patent application WO97/05691 which presents a solution for controlling the servomotor by a PWM converter and the switches of the converter are controlled so that the windings are short-circuited during the braking period. In the prior art references, the asynchronous motors with short-circuited rotors do not produce any torque in zero speed, because there is no rotating magnetic field during in the rotors.
In propulsion units according to prior art, braking, means operating mechanically have been devised. The object of the braking means is to prevent rotation of the propeller and to restrain the propeller essentially in a standard position or, respectively, to prevent the turning movement of the propulsion unit and to restrain the propulsion unit essentially in a standard position.